There is a nutation gear set as one of reduction gears. The nutation gear set is, for example, described in Patent Document 1, or the like. That is, the nutation gear set is formed of a first gear, a second gear and an input shaft that have the same rotation central axis, and a nutation gear that performs differential rotation with respect to the first gear and the second gear while wobbling therebetween. The nutation gear is supported by the input shaft so as to be rotatable about an inclined rotation central axis. Furthermore, the inclined rotation central axis relatively rotates about the rotation central axis of the first gear with the rotation of the input shaft. By so doing, the nutation gear wobbles with respect to the first gear and the second gear. Then, first nutation teeth that mesh with the first gear are formed on a face of the nutation gear, adjacent to the first gear, and second nutation teeth that mesh with the second gear are formed on a face of the nutation gear, adjacent to the second gear. Then, as the nutation gear wobbles, differential rotation occurs between the first gear and the nutation gear or between the second gear and the nutation gear. That is, when the second gear is set as an output shaft with respect to the input shaft, speed may be reduced at a large reduction gear ratio.
The nutation gear has an extremely complex meshing face that meshes with the first gear or the second gear, so machining is not easy. A machining device for the nutation gear is, for example, described in Patent Document 1.